The invention relates to a decanter-type separating apparatus comprising a rotatably mounted conico-cylindrical barrel and a screw-type rotor rotating and transporting solids inside said barrel at a speed differing from the speed of barrel rotation.
This type of decanting apparatus, which is referred to in the art as decanter centrifuge or, briefly, as a decanter, is commonly known and in most instances operates according to the countercurrent principle.
A particular design problem of decanter apparatus is the structure of the solids transporting screw. The geometry of the screw must be such that the solids may be discharged reliably from the tapered barrel section without sliding back into or having rotation imparted thereto by the barrel, instead of being discharged continuously.
It has been known from practical experience that the problems of solids transport and discharge increase with the taper of the barrel.
Also, it has been known that solids transport will improve with smaller pitches of the screw or flights on the rotor.
As a matter of experience, a taper of about eight to ten degrees has been found to be satisfactory.
Also, for the pitch of the screw, relative to the radius of the cylinder, a value within the range of six to eight degrees has been found to represent a suitable compromise between the various constructional and process requirements.
Major problems frequently arise in the area between the tapered and the cylindrical barrel sections. In that area, solids settle to form a cake having a substantially solid consistency in the cylindrical section. The solids cake has to be broken up when passing the joint between the cylindrical and the tapered sections and has to be given a new form adapted to the taper. Also, and particularly when treating paste-like products, solids may happen to accumulate in the tapered section and far back into the cylindrical section, although a pushing effect produced by the rotor will ultimately succeed in conveying the solids way up into the tapered barrel section. For the reasons briefly explained above, an effect may occur in the transition area from the cylindrical to the tapered barrel sections and, frequently, within the cylindrical barrel section itself which may be referred to as a "circulating solids transport", with the solids revolving with the barrel. This phenomenon of course greatly interferes with proper solids discharge and may disrupt it completely.
As it had turned out in the course of time that pitch angles up to about eight degrees in the tapered barrel section produce useful results, the conclusion was drawn that corresponding pitch angles are equally suitable for the cylindrical section of the barrel.
In general, those skilled in the art accepted this conclusion for a useful solution because there is a variety of reasons for which experiments involving full-size screw-type rotors are not feasible--particularly because of the exceedingly high fabrication costs of a screw, as manufacture of a useful screw-type rotor constitutes a major engineering and production problem and thus is quite expensive.
Of course, in many instances, the performance of known decanter-type separation apparatus was found not be satisfactory, and attempts have been made to find ways and means to improve on the clarification of the liquid.
Good clarification requires a substantial dwell time of the suspension on the rotor of a decanter. Also, the design must be such that disturbances are prevented in the flow of the liquid running off the flights.
As a result, the conditions to be fulfilled are a small differential speed between the screw and the barrel, slopes as smooth as possible of the flights, precise and uniform planar slopes of the screw even where the helix is assembled of sections welded together, and similar production requirements.
Of particular importance for stabilizing flow is a low backflow velocity. For a given liquid throughput, it would appear reasonable to select screw ducts as broad as possible, which would be theoretically feasible by increasing the pitch. Those attempts would be inconsistent with experience, however, and with the well-founded fear that it would not be possible any more to properly conduct the solids transport.
For this reason, those skilled in the art abstained from using greater screw pitches than about six to eight degrees. Instead, attempts were made to enhance clarification by improving flow conditions by other means.
In order to avoid flow disturbances caused by the screw, a short-circuit path or bypass was offered to the centrate, i.e. the centrate did not have to flow to the screw ducts any more.
Also attempts at obtaining improved results were made by using a so-called immersed screw rotor. The basic idea underlying the immersed screw rotor is to have the liquid or at least a portion thereof run to the overflow on a short-circuit or bypass path in a strongly perforated rotor body in order to limit flow and sedimentation disturbances to the screw space and to prevent them from affecting the stabilized zone in the rotor cavity. A large rotor body was used, as were narrow flights, and the centrate was allowed to flow to the overflow edge through perforations in the rotor body itself. The disadvantage of this approach, as it turned out to be, was that when filling the rotor during start-up, sedimented solids would drop into the rotor cavity and form localized deposits. When re-starting the rotor, those localized deposits would unbalance the decanter and force the apparatus to be disassembled and the rotor to be removed for cleaning.
Another attempt was to improve a ribbon-type screw design by axially guiding the liquid between the ribbon and the rotor body by means of ledges, ridges or similar channel-forming elements.
These additions presented complications in design and manufacture as well as difficulties to the servicing and maintenance effort. As a consequence, they never gained much practical significance.